The present invention relates to an elastomer composition suitable for use in a tread of tires, and more particularly to an elastomer composition containing a mixture of a high molecular weight styrene-butadiene rubber having a specific microstructure and a low molecular weight styrene-butadiene and/or butadiene rubber.
In recent years, reduction of running fuel cost of automobiles is strongly demanded from the viewpoint of energy saving. Therefore, it is of urgent necessity for the tire manufacturing industry to decrease the rolling resistance of tires, thereby decreasing the consumption of travelling energy.
Among the constituent elements of a tire, a tread rubber accounts for the largest proportion of the travelling resistance attended on rolling of the tire, and it is known that the tread rubber accounts for about 40% of the rolling resistance of the tire at large. Further, the motions of the tread rubber in travelling are classified into a bending and shearing motion and a compressive motion, and from the viewpoint of the viscoelastic characteristic of a rubber, the former is related to the loss modulus (E") and the latter is related to the loss compliance [E"/(E*).sup.2 where E* is complex modulus]. Further, these motions are repeated motions attended on rotation of tire, and the frequency thereof is from 10 to 10.sup.2 Hz. Accordingly, it is necessary for the reduction of the rolling resistance of tire to decrease the loss modulus and loss compliance of a tread rubber.
On the other hand, considering from the safety side the wet grip characteristic on a wet road which is the most important characteristic for tires and is an essential characteristic to travelling performances such as steering stability, breaking performance and turning performance on a wet road such as a road at the time of raining, it is a frictional characteristic caused by contact of the tread surface of a tire with the road. The frictional resistance is caused by viscoelastic behavior of a rubber in the surface of the tread against the stress that the rubber receives from the road, namely hysteresis loss due to phase lag. This is related to the loss coefficient (tan .delta.) from the viewpoint of the viscoelastic characteristic. Further, the frequency of the stress caused by the road is a very high frequency attended on the unevenness of road, and according to a literature, the frequency is from 10.sup.4 to 10.sup.7 Hz. Therefore, in order to improve the wet grip characteristic of tires, it is necessary to raise the loss coefficient of a tread rubber in 10.sup.4 to 10.sup.7 Hz.
In a tread rubber compound generally and widely used heretofore, natural rubber, synthetic isoprene rubber, synthetic butadiene rubber and styrene-butadiene rubber are mainly used as an elastomer component. The former three are superior in rolling resistance, but are poor in wet grip characteristic. Also, the latter one is superior in wet grip characteristic, but is poor in rolling resistance. Therefore, a tread rubber composition having excellent both properties of the rolling resistance and the wet grip characteristic has not been obtained. A tread rubber composition has been used at the cost of either one property or with keeping balance of the both properties by blending the above-mentioned elastomers.
The present inventors made a study of the repugnant properties, namely rolling resistance and wet grip characteristic, of conventional elastomers as mentioned above, and found as disclosed in Japanese Patent Application No. 157927/1981 that the rolling resistance is related to the viscoelasticity at a frequency of 10 Hz at a temperature of 50.degree. to 65.degree. C. and the wet grip characteristic is related to the viscoelasticity at a frequency of 10 Hz and a temperature of -30.degree. to -15.degree. C. The temperature range of -30.degree. to -15.degree. C. is converted into a frequency ranging from 10.sup.5 to 10.sup.7 Hz on the basis of the temperature-frequency relationship, and it agrees with a value described in literatures.
Therefore, it is understood that elastomers having balanced both properties of the rolling resistance and wet grip characteristic are desirable to be those having small loss modulus (E") and loss compliance [E"/(E*).sup.2 ] at 10 Hz in a high temperature region (50.degree. to 65.degree. C.) and having a large loss coefficient (tan .delta.) in a low temperature region (-30.degree. to -15.degree. C.). As elastomers having such a feature, the present inventors propose some elastomers as disclosed in Japanese Unexamined Patent Publication No. 65736/1982, No. 87444/1982, No. 94028/1982 and No. 123236/1982. Also, elastomers similar to such elastomers are disclosed in Japanese Unexamined Patent Publication No. 62248/1979, No. 12133/1980 and No. 143209/1981. These elastomers have well-balanced rolling resistance and wet grip characteristics, but are inferior to conventional elastomers with respect to other properties, e.g. processability in kneading, extruding and molding which is the most important in production of tires, and strength of elastomer such as abrasion resistance and durability.